let point = { x: 1, y: 1 };
console.log(`1: ${"x" in point}`); // 1: true
console.log(`2: ${"y" in point}`); // 2: true
console.log(`3: ${"z" in point}`); // 3: false
console.log(`4: ${"toString" in point}`); // 4: true
let data = [7, 8, 9];
console.log(`5: ${"0" in data}`); // 5: true
console.log(`6: ${1 in data}`); // 6: true
console.log(`7: ${3 in data}`); // 7: false
